The invention relates to a rolling die, in particular a flat rolling die for forming a screw thread on a screw blank. Such a rolling die has a plurality of grooves, each groove having two flanks in the cross-section.
In rolling, in particular in flat die rolling of thread profiles, material is pushed together from opposing groove flanks in the area between the thread pitches of the screw blank on both sides until the thread profile has been completely formed. To this end, European Patent Document No. EP 0 533 456 B1, for example, describes a rolling die. With the rolling die according to EP 0 533 456 B1, two flank areas with profile angles of different sizes are superimposed in the grooves, such that the height ratio of the two flank areas is varied along the grooves. Additional rolling dies having two flank areas with different profile angles are known from German Patent Document No. DE 1 283 791 A and U.S. Pat. No. 3,069,941.
Based on the dependencies between the required thread profile angle of the screw and the profile angle in the forming area of the rolling die, the displacement of material during rolling may not be optimal with the known rolling dies under some circumstances, which may be reflected in uneven die load. Under some circumstances, this may have a negative influence on the lifetime of dies, production rates and thread quality, and comparatively long rolling dies may be required under some circumstances.
The object of the invention is to provide a rolling die with which an especially good screw quality can be obtained with especially high reliability, especially high production rates and especially low effort.
A rolling die according to the invention is characterized in that at least a portion of the grooves have a forming area, in which the size of a profile angle enclosed by the flanks decreases continuously along the grooves.
One basic idea of the invention is to design the grooves with a variable profile angle, which decreases continuously along the grooves, in particular from the inlet area of the grooves to the calibration area of the grooves. In contrast with the dies according to EP 0 533 456 B1 and DE 1 283 791 A, in which the thread pitch of the blank is acted upon first at a first angle and in the further course of forming is acted upon directly at a second angle, at which the angle of the attacking flank on the workpiece is varied discontinuously, the angle of the attacking flank changes continuously according to the invention, so that the stress on the material of the flank can be distributed especially uniformly over the forming operation according to the invention. In particular through the type of die design according to the invention, it is possible to achieve the result that the thread precursors are already constructed near the inlet area at an early stage, so that the highly stressed calibration area is relieved and thus the lifetime of the dies is increased. In addition, it has been found that when using rolling dies according to the invention, the depth of the closing fold between the bulges of material pushed up by the neighboring webs may be reduced by improved material displacement, so that the quality of the thread is increased. Finally, it has been found that the invention allows the production of screws with a large screw pitch and at the same time a great thread flank height without having to significantly increase the length of the die and without any mentionable loss of production rate.
For example, the profile angle α may be 80° to 120° in the inlet area. The size of the profile angle α changes to the value of the desired thread profile, i.e., preferably changing to values between 30° and 60° as it changes from large to small over the forming area.
According to the invention, the grooves are created in a planar working face of the rolling dies, for example. According to the invention, the grooves run at least approximately parallel to one another. Thread forming webs are formed between the grooves. Material of the workpiece is pushed together by the webs on both sides on the flanks of the grooves until the thread profile has been formed. The teaching according to the invention, i.e., that the size of the profile angle decreases continuously along the grooves, may include in particular the fact that the profile angle becomes smaller and smaller with an increase in the distance along the grooves, such that this reduction does not take place suddenly but instead is continuous.
In a particularly simple embodiment, it is possible to provide that the size of the profile angle decreases linearly along the grooves in the forming area. It is especially preferred for the size of the profile angle to decrease hyperbolically along the grooves in the forming area. Accordingly, the profile angle follows a predetermined linear and/or hyperbolic function; this function includes the path along the grooves. This makes it possible to further increase the quality of the thread.
Furthermore, it is advantageous that the width of at least a portion of the grooves is varied along the grooves, preferably hyperbolically in the forming area. Accordingly, the width follows a predetermined function, preferably hyperbolic, in which the path along the grooves is a factor. It is possible in particular to provide that the width of at least a portion of the grooves decreases continuously along the grooves in the forming area. The quality of the thread can be further increased. Both the profile angle and the width of the grooves are expediently varied in the manner described here in at least a portion of the grooves.
A further improvement in reducing the depth of the closing fold in particular can be achieved by the fact that the profile depth Z of the grooves is adapted to the enclosed volume at each location in the forming area, so that an influence of the lengthening of the bolt during rolling must be taken into account under some circumstances.
Another preferred embodiment of the invention consists of the fact that at least a portion of the grooves have a foot area with a foot area profile angle and have a head area with a head area profile angle in at least a portion of the forming area in the cross-section, such that the foot area profile angle is expediently smaller than the head area profile angle in at least some areas and such that at least the size of the foot area profile angle decreases continuously along the grooves, preferably in the forming area. According to this embodiment, the profile angle according to the invention, which is referred to here as the foot area profile angle, is superimposed on an auxiliary angle in the area of the opening of the groove, this auxiliary angle being referred to here as a head area profile angle. The head area profile angle, i.e., the auxiliary angle, may be constant or variable along the grooves. Such an auxiliary angle may produce an even more improved flow of material into the thread flank due to an even larger initial angle, e.g., to achieve an especially good forming of the thread in the case of large thread pitches and large thread flank heights. The auxiliary angle (head area profile angle) may follow a predetermined function, the course of which may be defined as a function of the foot area profile angle α. The auxiliary angle (head area profile angle) γ may be, for example, between 100° and 140°. The depth of the head area may be designed to be variable over the forming area. Suitable values for the depth of the head area may be 0.1 to 1 mm, for example.
It is also expedient that a pitch angle ω between the grooves and the longitudinal axis of the rolling die is varied along the grooves at least in the forming area. Accordingly, the pitch angle follows a predetermined function, which is based on the path along the grooves. This embodiment provides that, in the case of screw profiles that cause a relatively great change in the rolling circle during rolling due to the geometric factors, the angle between the grooves and the longitudinal axis of the rolling die is not constant but instead is advantageously adapted to the actual prevailing rolling circle at each point, which thus entails a non-constant course of the angle.
The invention also relates to a method for forming a screw thread on a screw blank, in which the screw blank is formed by means of at least one rolling die according to the invention, and the screw thread is thereby formed, as well as relating to the screw produced in this way. The screw blank is preferably formed between two rolling dies.
According to the invention, it is possible to provide for all parameters which determine the geometry of the grooves to be designed to be variable over the entire forming area of the rolling die. The variable parameters for designing the wedge-shaped grooves may be in particular: α (profile angle), a (groove width) and Z (groove depth). The initial values of the parameters are defined according to the requirement of the desired thread profile and follow over the entire forming area of a defined function, expediently as a continuous function, preferably taking into account the constant volume. At the end of the forming area and/or in the calibration area, the parameters correspond to the desired thread profile. The profile angle α and the groove width a preferably follow a hyperbolic function, which may be defined according to the requirements of the desired thread profile. The groove depth Z is defined as a function of the design of the profile angle α and the groove width a. Due to the variable design of the profile angle α and the groove width a over the forming area, it is possible to generate an optimal displacement of material, depending on the requirement of the desired thread profile.
In addition, the grooves in the forming area may be designed with a variable pitch angle ω to the longitudinal axis of the die. In the case of thread profiles having a large thread pitch and a large thread flank height in particular, causing a relatively great change in the rolling circle during rolling due to the geometry, it is possible to optimize the displacement of material into the thread flank through the variable design of the pitch angle to the longitudinal axis of the die.
The variable design of the profile parameters according to the invention offers more possibilities in the production of screw threads with a large thread pitch on the one hand and with a large thread flank height at the same time without increasing the die length and/or the forming area, while on the other hand relieving the load on the forming area in the vicinity of the calibration area, which contributes toward an increase in the lifetime of the die. This increases the technical production possibilities in existing capacities, where the production rate can be kept at a higher level.
The invention is explained in greater detail below on the basis of preferred exemplary embodiments, which are shown schematically in the accompanying figures.